Wire dot printer with improved wire dot head

ABSTRACT

A wire dot printer has a wire dot head comprising a plurality of electromagnets for attracting each armature to thereby project each wire to a printing position, a first cover provided with a guide for guiding the wires to the printing position, holding means for holding the electromagnets, the holding means being formed of a material of high heat radiation efficiency, and a second cover connected to the holding means with high heat conducting efficiency and covering the electromagnets and the armatures, the second cover being provided with radiation fins at a plurality of locations thereon.

This application is a continuation of application Ser. No. 531,336,filed Sept. 12, 1983, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a wire dot printer in which wires areselectively projected by a plurality of electromagnets to effectprinting (including recording).

2. Description of the Prior Art

When it is desired to form a character by a dot matrix, if it is assumedthat an m×n dot matrix is disposed as shown in FIG. 1 of theaccompanying drawings, use is made of a printing head comprising n wiresarranged in a row at a pitch P₂ in the longitudinal direction and, ifthis printing head is shifted to right or left m times at a pitch P₁ andthe wires are designed so as to be capable of being driven for eachpitch, there can be formed m×n dot matrices.

Generally, a wire dot head is such that an armature is attracted andoperated by an electromagnet contained in the head body and a wire isurged against printing paper by the armature through an ink ribbon,whereby desired printing is effected by the aforementioned dot matrices.When the armature is attracted and operated, heat is generated byexcitation of the coil of the electromagnet and therefore, the head bodyheats. Accordingly, exciting current is decreased by the increase incoil resistance caused by this heating and the force whichelectromagnetically attracts the armature is reduced and the operationefficiency of the armature is reduced. Therefore, the urging driveforce, i.e., the printing pressure, of the wire connected to thearmature becomes small, thus resulting in a reduced quality of printing.Also, the surface temperature of the head rises to the order of 90° C.,and this is very dangerous to the operator.

Also, a wire dot printer effects printing by impact and this leads tothe advantage that several copies can be produced at a time, but as adisadvantage thereof, noise is great. Accordingly, such noise haslimited the use of conventional wire dot printers in offices. There aretwo causes of the sound produced when printing is effected by the use ofsuch a wire dot printer. A first cause is the impact sound produced whendots are printed on printing paper. The quality of such impact sound isdetermined by the material and hardness of the platen, the mountedcondition of the printing paper and the printing force of the wire. Asecond cause is the stop sound produced when the armature to which thewire is connected is stopped by a stopper in the course of its return tothe original position after completion of printing.

In the wire dot printer, not only noise produced by such collision ofthe wire, but also the operation of the armature becomes unstable due tothe creation of rebound of the armature, and this has led to theshortcoming that the quality of printing is deteriorated.

Thus, the conventional wire dot printers have not always been sufficientin the operation efficiency of the armature.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a wire dot printerof high operation efficiency.

It is another object of the present invention to enhance the radiationefficiency of heat generated from electromagnets.

It is still another object to facilitate the assembly of a wire dothead.

It is yet still another object of the present invention to reduce thenoise level.

It is a further object of the present invention to effectively effectthe radiation from radiation fins by utilizing the movement of the wiredot head in the printing direction.

It is still a further object of the present invention to stabilize theoperation of armatures.

It is yet still a further object of the present invention to reduce thenumber of parts.

It is a further object of the present invention to eliminate the reboundof the armatures.

Other objects will become apparent from the following detaileddescription taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view showing the construction of dot matrices;

FIG. 2 is a longitudinal cross-sectional view showing an embodiment ofthe wire dot head according to the present invention;

FIG. 3 is an exploded perspective view of the essential portions of thewire dot head shown in FIG. 2;

FIG. 4 is a view taken along line A--A' of FIG. 2;

FIG. 5 is a view-taken along line B--B' of FIG. 2;

FIG. 6 is a partly cross-sectional view for explaining the conduction ofheat from an electromagnet in the embodiment shown in FIG. 2;

FIG. 7 schematically illustrates a vibration system;

FIG. 8 is a characteristic graph showing the transmission factor of thedisplacement from a stopper to a damper;

FIGS. 9(A) and (B) illustrate the operation of the armature when aconventional stopper is used;

FIGS. 10(A) and (B) illustrate the operation of the armature when thestopper of the present invention is used; and

FIG. 11 is a perspective view of another embodiment in which the stopperand the armatures are made integral with each other.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 2 is a longitudinal cross-sectional view of a wire dot head used ina wire dot printer in accordance with the present invention, and FIG. 3is an exploded perspective view of the essential portions thereof. Inthese Figures, reference numeral 1 designates a tip end guide forfacilitating the assembly of a wire 22 to the head body, and referencenumeral 2 denotes an intermediate guide mounted on a back holder (to bedescribed) by screws 23 through a mating portion for positioning, notshown, and serving also as a cover for covering the forward portion ofan internal electromagnet. The intermediate guide 2 is formed withgrooves 2a for screws for attaching it to a carriage, not shown, and isU-shaped for the ease of mounting and dismounting of the head. Referencenumeral 3 designates a flexible cable for supplying a current to thecoil of an electromagnet to be described, and reference numeral 4denotes a radiator plate formed of a material of good heat conductivitysuch as aluminum, the radiator plate 4 making surface contact with theyoke of an electromagnet to be described and a back holder to bedescribed and serving also as a spacer between the aforementionedflexible cable 3 and the yoke. In the present embodiment, the thicknessof the radiator plate 4 is about 2 mm to enable the heat of the yoke tobe sufficiently conducted to the back holder, and the material of theradiator plate is preferably aluminum or magnesium which has good heatconductivity. In the present embodiment, aluminum is employed as thematerial of the radiator plate. Reference numeral 5 designates the yokeof the electromagnet. The yoke 5 is formed into an elliptical shape asshown in FIG. 4 and is designed to unify the sliding resistance of thewire and prevent the density irregularity between dots formed onprinting paper. Designated by 6 is a core coupled to the yoke 5. Abobbin 8 having a coil 7 wound thereon is fitted on the core 6. Thebobbin 8 is provided with two terminals 8a. The beginning end and theterminal end of the coil 7 are soldered to the two terminals 8a, whichis turn are soldered to the aforementioned flexible cable 3 to permitthe supply of power to the coil 7. Reference numeral 9 denotes a rearguide positioned at the center of the head body and contained in theyoke 5. One end of a restitution spring 10 for returning an armature 14to its initial position after it is driven is secured to the rear guide9. The armature 14 is attracted to print dots by the wire 22, whereafterit is returned to its initial position by the restitution spring 10 anda stopper 15. The restitution spring 10 and the stopper 15 are alignedwith each other so that the movement of the armature 14 is stable. Thatis, the strength of the restitution spring 10 and the hardness of thestopper 15 are determined so that stable movement of the armature 14 ispossible and, in the present embodiment, polyurethane or the like isemployed as the material of the stopper 15.

Reference numeral 11 designates an auxiliary yoke juxtaposed with theyoke 5, and reference numeral 12 denotes a spacer of polyester film orthe like interposed between the auxiliary yoke 11 and the armature 14 toprevent the instability of the movement of armature 14 based on theabrasion resulting from the rotation caused by the line contact with theauxiliary yoke 11 and the residual magnetism of the auxiliary yoke 11which offers a problem during the formation of a closed magneticcircuit. Designated by 13 is a plunger secured to the armature 14. Awire 22 is coupled to one end of the armature 14 by brazing. Anaperture, not shown, in which the mating portion of the bobbin 8 isfitted is provided at the other end of the armature 14, and the armature14 is pivotable with this mating portion as the fulcrum. Denoted by 16is an armature holder which holds the pivot portion of the armature 14from the rear thereof during the pivotal movement of the armature tothereby achieve the stabilization of the pivotal movement of thearmature 14. The aforementioned stopper 15 is fitted to the armatureholder 16. Reference numeral 17 designates a plate spring provided incontact with the armature holder 16, and the armature holder 16 isuniformly urged by resilient displacement of this plate spring 17. Thearrangement relation of the auxiliary yoke 11, armature 14, armatureholder 16 and plate spring 17 is shown in FIG. 5. Reference numeral 18denotes a damper provided on the plate spring 17. This damper 18 isurged and held by a back holder 21 formed of a material having high heatconductivity such as aluminum and functioning as an extraneous radiatormember, through a washer 19 and a spring washer 20. The back holder 21is of a shape which serves also as a cover for covering the rear of aninternal electromagnet and which has a plurality of planar fins 21aintegral with one another to enhance the radiation effect. These fins21a, as clearly shown in FIGS. 2 and 3, are formed so that theirlengthwise direction is a direction substantially perpendicularlyintersecting the direction of arrangement of the wire 22 arrangedsubstantially rectilinearly in the longitudinal direction from thecenter thereof to the fore end thereof. This is because the wire dothead is fixed to a carriage so that each end of the wire 22 is arrangedin a direction substantially orthogonal to the direction of movement ofthe carriage when the wire dot head is mounted on the carriage andeffects printing, and also because if these radiation fins are arrangedlengthwisely in a direction orthogonal to the direction of arrangementof the wire 22, namely, a direction substantially parallel to thedirection of movement of the carriage, the radiation effect of the finsis very effectively provided by movement of the carriage. Also, the backholder 21 and the intermediate guide 2 have no unnecessary openings sothat the interior thereof is substantially hermetically sealed when theyare fitted and fixed by screws 23 at the opposite sides and internalnoise cannot escape directly.

On the other hand, the damper 18 is designed to further enhance itsanti-vibration effect by being pivotably provided by the plate spring 17and the spring washer 20. Also, the washer 19 prevents the spring washer20 from abrading the damper 18. The wire 22 is designed to be supportedby the guide 1, 2 and 9 for the ease of the assembly thereof, to reducethe drag from each guide, to decrease the sliding resistance of the wire22 and to improve the efficiency of converting electrical energy intoprinting energy.

When power is supplied to the coil 7 of the head of the above-describedconstruction through the flexible cable 3, a closed magnetic circuit isformed by the yoke 5, auxiliary yoke 11, plunger 13 and core 6 andtherefore, the plunger 13 is attracted to the core 6 with a result thatthe wire 22 coupled to the armature 14 is pushed out and dots are formedon printing paper through an ink ribbon, not shown. As previouslydescribed, however, the coil 7 heats due to the resistance of the coil 7itself and the generation of an eddy current in the core 6 and theattraction is decreased by the decrease in exciting current resultingfrom an increase in resistance of the coil 7 itself and this may resultin reduced quality of printing. In this case, according to theexperiment, the heat-resisting temperature of the coating of the coil 7is about 120° C. and the heat-resisting temperature of the bobbin 8 isabout 150° C. and therefore, after all, the temperature of the solenoidportion must be maintained below 120° C. To this end, a mechanism forenhancing the efficiency of converting electrical energy into printingenergy and enhancing the radiation of the heat generated must beprovided. As the countermeasure for the former, means may be providedwhich will enhance the magnetic efficiency of the magnetic circuit andmoreover minimize the sliding resistance of the wire 22, and as thecountermeasure for the latter, means may be provided which will enhancethe radiation of heat.

According to the present embodiment, as will be seen from the radiationpath of heat from the electromagnet of FIG. 6, the heat generated in thecoil 7 and core 6 is collected into the yoke 5 to which the core 6 iscoupled from each solenoid portion constituting the electromagnet, andthe yoke 5 in heating condition is in intimate contact with the radiatorplate 4 and therefore, the heat thereof is efficiently conducted to theradiator plate 4. The heat thus conducted to the radiator plate 4 isfurther conducted to the back holder 21 through the portion which is inintimate contact with the back holder 21 and which functions as theaforementioned external radiator member.

According to an embodiment of the present invention, the heat from thecoil 7 and core 6 is efficiently radiated through the back holder 21 andmoreover, this back holder is very excellent in radiation efficiency andthus, reduction in printing force does not occur and high quality ofprinting can be maintained even if printing is effected for a long time.Also, by reduction of the temperature of the wire dot head by about 20°C. can be achieved by this radiator mechanism and safe printingoperation becomes possible.

The damper 18 will now be described in detail.

The armature 14 after having completed the printing of each dot is movedto its initial position by the restitution spring 10 and stopped thereatby bearing against the stopper 15. This stopper 15 is made of rubber andtherefore, the shock vibration caused when the armature 14 is stopped istransmitted to the armature holder 16 through the stopper 15. The shockvibration transmitted to the armature holder 16 is further transmittedto the plate spring 17 and the damper 18. The vibration system in thiscase is schematically shown in FIG. 7, wherein m is the mass of thedamper 18, x is the displacement of the damper 18, c and k are theviscosity attenuation coefficient and the spring constant, respectively,of this vibration system. The dotted line box represents the stopper 15.If the displacement thereof is u=A sin ωt and the frequency of thearmature 14 is f,

    f=(ω/2π)                                          (1),

where ω is called the angular frequency. In the case of the presentembodiment, the frequency of the armature is f=800 (Hz).

If the displacement of the damper 18 is x when forcible displacement uacts on the stopper 15, the displacement of the spring and of theattenuator is equal to the relative displacement (x-u) between thedamper 18 and the stopper 15. Accordingly, the movement equation of thevibration system is:

    mx=-c(x-u)-k(x-u)                                          (2)

    mx+cx+kx=cu+ku                                             (3)

where, from u=A sin ωt,

    mx+cx+kx=A(k sin ωt+cωcos ωt)=A√k.sup.2 +(cω).sup.2 sin (ωt+α)                  (4),

where α is the phase angle and

    α=tan.sup.-1 (cω/k)                            (5)

The steady vibration of the damper 18 is equal to the vibration causedwhen a harmonic vibromotive force indicated by A√k² +(cω)² sin (ωt+α)directly acts on the damper 18, and the amplitude thereof is ##EQU1##Also, the ratio of the vibration of the damper 18 to the vibration ofthe stopper 15 is ##EQU2## where T_(R) is the transmission factor ofdisplacement, and ξ is the viscosity attenuation percentage and isexpressed by ##EQU3## Also, ##EQU4##

FIG. 8 shows the magnitude of the transmission factor of displacement.As seen in FIG. 8, irrespective of the attenuation percentage ξ,T_(R) >1 when ω/ωn>√2, T_(R) ≦1 only when ω/ωn≧2. Generally, the effectof vibration isolation is better as the natural frequency of thevibration system is smaller. That is, as can be seen from equation (9),either the mass of the damper 18 can be made great or the springconstant can be made small. To make the mass of the damper 18 great, usemay be made of a material having a great specific gravity and, in thecase of the head of the present embodiment, lead is used to make themass great. On the other hand, to make the spring constant small, in thehead of the present embodiment, the damper 18 is sandwiched between theplate spring 17 and the spring washer 20.

Thus, according to an embodiment of the present invention, as can beseen from equations (8) and (9) and FIG. 8, the material and the holdingstructure of the damper 18 are such that the transmission factor ofdisplacement is made small and therefore, the stop sound produced whenthe armature 14 strikes against the stopper 15 can be weakened, and thisleads to realization of a low noise wire dot head. By so using thedamper 18, the plate spring 17 formed of stainless steel and the springwasher 20 around the stopper member for the armature, the stop sound ofthe armature 14 can be reduced and, according to the experiment,reduction of noise by about 10 dB can be achieved as compared with theconventional wire dot head.

The stopper 15 will now be described in detail.

When power is supplied to the coil 7 of the wire dot head of theabove-described construction, a closed magnetic circuit is formed by theyoke 5, auxiliary yoke 11, plunger 13 and core 6 and therefore, theplunger 13 is attracted toward the core 6 and the armature 14 pivots soas to push out the wire 22 against the force of the restitution spring10. At this time, the wire 22 prints dots on printing paper through anink ribbon. At this moment of printing, the wire 22 restores itsoriginal position by the repulsing force from the platen and thearmature 14 restores its original position by the force of therestitution spring 10 and bears against the stopper 15, thereby beingstopped at its initial position. At this time, the armature 14 effects asubtle movement due to the stopper 15 and the restitution spring 10.That is, the material and hardness of the stopper 15 and the springconstant of the restitution spring 10 determine the movement of thearmature 14. If the spring constant of the restitution spring 10 isgreat, the attraction is decreased and therefore the printing force isalso reduced, but the armature 14 is easy to restore its initialposition in a stable condition.

On the other hand, if the hardness of the stopper 15 is high, thearmature 14 becomes liable to repel from the stopper 15. If the hardnessof the stopper 15 is low, the armature 14 rebounds less from the stopper15 and movement of the armature 14 becomes stable.

The aforementioned state of unstable movement of the armature 14 isshown in FIGS. 9(A) and (B). FIG. 9(A) shows the relation between thedisplacement x of the end of the wire by defining the non-dimensionalparameter x/c as x divided by the the clearance c between the end of thewire and the end of the printing head, and time, printing being effectedat x/c=1.0. On the other hand, FIG. 9(B) shows the relation betweentransmission signal (voltage) and time. FIGS. 9(A) and 9(B) correspondto each other in respect of their time axes. That is, where the periodof the signal is T and the power supply time is τ, the wire 22 begins tomove from a with a delay with respect to the signal and arrives at thesurface of paper at b and effects printing, whereafter the wire 22 isreturned to the position of the stopper 15 by the repulsing force fromthe platen and the force of the restitution spring 10. However, due tothe inappropriate spring constant of the restitution spring 10 or thehigh hardness of the stopper 15, the wire 22 rebounds at c from thestopper 15 and moves to d. At this time, the wire 22 tries to return toits original position with the aid of the force of the restitutionspring 10, but the next signal is supplied at e and therefore, the wire22 tries to move toward the surface of paper and arrives at the surfaceof paper at f, thus printing dots. At this point e, the attractiontoward the surface of paper is reduced by the inertia force of the wire22 and therefore, the printing force is reduced at the printing point fand the printed dots become thin. Such unstable operation occurs even ifthe spring constant of the restitution spring 10 is set appropriatelybut if the hardness of the stopper 15 is high. In the conventional head,use has been made of a stopper 15 of plastics having a high hardness andtherefore, movement of the wire 22 has become unstable and also greatshock sound has been produced when the armature 14 bears against thestopper 15.

In the present embodiment, the restitution spring 10 used is of anappropriate spring constant and made of polyurethane rubber. In thepresent embodiment, the hardness of the rubber is 94° C., and polyesterand isocyanate are mixed with polyurethane rubber to improve thewearability of the restitution spring and prevent the firstnon-operation resulting from the intimate contact between the armature14 and the stopper 15. The use of such resilient stopper 15 causes thestopper 15 to be displaced when the armature 14 bears against thestopper 15 and therefore, the vibration energy is absorbed and the soundproduced is small. The resulting movement of the wire 22 for anelectrical signal is shown in FIGS. 10(A) and (B). As will be seen fromFIGS. 10(A) and (B), the wire 22 after printing quickly restores itsoriginal position and can maintain an operation corresponding to theelectrical signal because no rebound is created by the resilient stopper15.

An embodiment of the present invention has been described above. Sincethe member constituting the stopper 15 is formed of wear-proofpolyurethance rubber, the shock absorbing property relative to thearmature 14 is improved, and the unstable movement of the armature asshown in FIG. 9(A) is eliminated and therefore, the print densityirregularity and pitch irregularity during printing are prevented andthe quality of printing is remarkably improved. According to theexperiment, reduction of noise by about 10 dB becomes possible ascompared with a case where the conventional wire head is used.

Description will now be made of another embodiment, shown in FIG. 11, inwhich the stopper 15 and the armature holder 16 are made integral witheach other.

To prevent the reduction of the quality of printing caused by the wearof the supporting portion of the armature holder 16 which supports thearmature 14, the material of the armature holder 16 must be a wear-proofmaterial. Accordingly, in the present embodiment, the material of thearmature holder 16 is the same as the material of the stopper 15.

That is, in the present embodiment, use is made of a restitution spring10 of an appropriate spring constant and the stopper 15 and the armatureholder are formed integrally with each other and the material thereof ispolyurethane rubber. This polyurethane rubber, like that in thepreviously described embodiment, has polyester and isocyanate mixedtherewith.

The use of such stopper 15 and armature holder 16 which are madeintegral with each other results in not only the improved shockabsorbing property of the stopper 15 relative to the armature 14, butalso improved wearability of the armature holder 16, which in turn leadsto unified urging force imparted to the armature 14 and accordingly tostabilized operation of the armature 14. Thus, the operation of the wire22 also becomes stable and maintenance of a high quality of printingbecomes possible.

Further, in addition to using a common material for the stopper 15 andthe armature holder 16, making the stopper 15 and the armature holder 16integral with each other as shown in FIG. 11 reduces the number of partsand improves the assembly efficiency.

In the present invention, the stopper 15 and the armature holder 16 maybe made separate from each other and moreover, these two members may beformed of a material composed of chiefly polyurethane rubber havingpolyester and isocyanate mixed therewith.

What I claim is:
 1. A wire dot printer having a wire dot headcomprising:a plurality of armatures provided correspondingly to aplurality of wires and held so as to be reciprocally pivotable about afulcrum; an electromagnet for attracting said armatures for pivotablymoving said armatures and driving said wires; a stopper formed of amaterial having elasticity for stopping said armatures when they pivotfor return; shock absorbing means for alleviating the shock with whichsaid armatures strike against stopper, said shock absorbing means formedof a material having a greater specific gravity than that of thematerial of said stopper, and said shock absorbing means beingresiliently urged by a spring member toward said stopper; and holdingmeans for pivotably holding said plurality of armatures, said holdingmeans being urged by said spring member to hold said armatures.
 2. Awire dot printer having a wire dot head comprising:a plurality ofarmatures provided correspondingly to a plurality of wires and held soas to be reciprocally pivotable about a fulcrum; an electromagnet forattracting said armatures for pivotably moving said armatures anddriving said wires; a stopper formed of a material having elasticity forstopping said armatures when they pivot for return; shock absorbingmeans for alleviating the shock with which said armatures strike againstsaid stopper, said shock absorbing means formed of a material having agreater specific gravity than that of the material of said stopper, andsaid shock absorbing means being resiliently urged by a spring membertoward said stopper; holding means for pivotably holding said pluralityof armatures, said holding means being urged by said spring member tohold said armatures; and a second spring member provided between saidshock absorbing means and said holding means to fully utilize an elasticforce thereof, the force to said spring member being transmitted to saidholding means through said second spring member.
 3. A wire dot printerhaving a wire dot head comprising:a plurality of armatures providedcorrespondingly to a plurality of wires and held so as to bereciprocally pivotable about a fulcrum; an electromagnet for attractingsaid armatures for pivotably moving said armatures and driving saidwires; a stopper formed of a material having elasticity for stoppingsaid armatures when they pivot for return; shock absorbing means foralleviating the shock with which said armatures strike against saidstopper, said shock absorbing means formed of a material having agreater specific gravity than that of the material of said stopper, andsaid shock absorbing means being resiliently urged by a spring membertoward said stopper; and another spring member for also urging againstsaid shock absorbing means, said shock absorbing means being sandwichedbetween said spring member and said another spring member.